At what point does a person actually die? That depends on who you ask. To one person, it's the moment the heart stops beating. To another, it's when the brain enters a "vegetative" state. But a heart can be forced to keep beating; and how dead is a person, really, if she can continue to grow, develop, and even give birth after experiencing "brain death"?

In your experience, do most doctors and scientists relate to death in similar terms? In other words, does "death" mean the same thing to a cardiologist as it does to a cell biologist, a neurologist, or a neonatologist?

I've been a science writer since 1973, covering a lot of particle physics, and I've discovered that compared to, say, physics, "medical science" is an oxymoron. Doctors are not well schooled in scientific principles. They are healers, not scientists, and they don't understand basic concepts such as falsification in science. For example, doctors believe that if drug A heals 9 out of 10 people with disease X, then drug A heals disease X because it usually does. A scientist, on the other hand, believes that one exception destroys the whole theory.

More to the point, take brain death. Some patients are declared brain dead and then begin spontaneously breathing hours later. Medical scientists say it doesn't matter because most brain-dead patients do not come back to life, but a rigorous scientist would say that these cases speak loudly about the flaws in our criteria for death. And yes — death to a cardiologist means that your heart has stopped, and he can't get it to restart. But to a neurologist, it might mean something else. In 1968, a committee at Harvard Medical School put forth an article stating that there is a second kind of death: brain death. Even though your heart is still pumping, and you're still able to breathe on a ventilator, if your brain stem is down, you're dead. This theory was made law in all 50 states in 1981, so now in the U.S. we have two kinds of death: real death (cardiopulmonary death) and what some doctors call "pretty dead," or brain death. A cell biologist, on the other hand, may have a standard more rigorous than cardiologists or neurologists. They might want to see all one's cells dead, which we call putrefaction.

So clearly death is not as straightforward as some people make it out to be. Tell us a little about why that is.

Since the beginning of recorded history, we have looked for a simple set of criteria that tell us when a person is dead. This is because we don't like to bury or cremate people if they're still alive, among other reasons. We have looked for a central organ that spells the difference between life and death, or a set of behaviors that indicate with certainty that our bodies have called it quits.

But every time we think we have solid criteria, we find exceptions. The ancient Egyptians, for example, thought the brain was totally unimportant, and they hollowed out the skulls of mummies, tossing the brain away. They concentrated on the heart. But stopped hearts often restarted spontaneously, and embalmers who declared live persons dead were stoned.

The Romans came up with "conclamation," which involved yelling a person's name in his ear three times — hardly a foolproof method. Inventions like the stethoscope helped immensely because some heartbeats and breaths are faint. Artificial respiration, smelling salts, and electric shock resuscitated people previously thought dead. Medical journals continue to fill with conditions that mimic death but which are not death at all.

Would you say that definitions of death have evolved over time?

Definitions of death have not changed a great deal, but that doesn't mean much. Often we'll say "death is the absence of life," but then we have to define "life" and that's almost impossible. All we can really do is set criteria for who's dead, and that is tough enough, and we have no real definitive set of standards.

You suggest in your book that relating to death as something that is "irreversible" can be problematic. Talk to us about what makes that word so troublesome.

"Irreversible" is not a very scientific term. Is the solar system "irreversible?" How about the universe as a whole? No, even the proton may eventually decay. Nothing is forever.

What science looks for is stable systems. We can say the solar system or the hydrogen atom is stable. Death comes, we might say, when the stability of the human body breaks down, and the system no longer works as a whole. Does that happen when the heart goes? The lungs? The lungs and heart together? The brain? That's the debate. We are acquiring more and more evidence that the body can go on in a somewhat stable system long after the brain has called it quits. For example, brain-dead pregnant mothers can continue to gestate and give birth to their babies long after being declared brain dead. In one case, a mother went 107 days after "death," and then delivered a healthy newborn.

"An MRI that twenty years ago was considered a 'photograph of death' is now just an image of a sick but reparable brain."

How do you think our understanding of death might continue to evolve in the years to come?

I can only hope that medical scientists become true scientists, and acknowledge some ugly truths: that life lingers on far past our criteria for death. Perhaps it is impractical for us to keep people alive-and on life support-indefinitely. Today we declare such inconvenient people to be "dead," even if they're not, so we can bury them and be rid of them. Perhaps we should acknowledge that they are not dead, but in, say, "condition X," a condition at which time we can terminate them. These are ugly realities, and it is easier just to call them dead. But we should face reality, and make tough decisions.

Do you think our relationship with death is more likely to change in response to breakthroughs in medical treatments, or advances in our ability to understand the body? I recognize that these two things are not mutually exclusive; but if I might borrow an example from your book, what I'm getting at here is the distinction between something like tissue plasminogen activator (a medical advancement that "moved the line" dividing life and death, so to speak) and imaging techniques like MRI that allow us to visualize a brain and decide if someone is or is not "dead."

Yes, [tissue plasminogen activator, aka "tPA"] is a good example. In the past, brain scans taken of people 1 to 3 hours after a stroke would be read as the scans of a dead person. Today, tPA can bring people back to normality even 3 hours after a stroke. So an MRI that twenty years ago was considered a "photograph of death" is now just an image of a sick but reparable brain.

But again, these are not definitions but criteria for death. One of the problems of brain death is that it was described as death all the way back in 1968, and there have been remarkable discoveries in neuroscience since then. In 1973, Candace Pert discovered the opiate receptor in the brain. The discovery of endorphins followed, as did dozens of other receptors and neurohormones. The brain of 1968 was envisioned as a Tinker Toy kind of machine, with electricity mixed in. Now we know biochemistry has a great deal to do with consciousness.

And yet, that has all been ignored because brain-death criteria were developed in the stone age of neuroscience, and neurologists still are schooled in this backward fashion. Neurochemicals are found being secreted in supposedly dead brains because brain-death tests are not designed to detect them. Our tests are crude, requiring an exam shorter than my last eye exam, and using such crude instruments as a flashlight, ice water, cotton swabs, and the like. None of this can tell us about consciousness.

Organ harvesting/transplantation and the science surrounding death. You suggest that the two have difficulty playing nice. Why is that?

Plain and simple: we want the organs. Organ transplantation is a $27 billion per year business. Most of the people harvested for organs today would not have been considered dead prior to 1968. But really dead people-those whose hearts have stopped and are not breathing-do not make good donors. The blood stops bringing oxygen to the organs, and, for lack of a better word, they spoil. But if the donor is just "mostly dead," meaning that his heart is still beating and he can still breathe with the aid of a ventilator, then the organs remain fresh and juicy and bring huge prices in the transplant business.

The average cost for a heart transplant, such as the one Dick Cheney just received, is nearly $1,000,000. Kidneys will cost you about $250,000 each. The donors by law cannot be compensated, so this is a very, very profitable business, one in which the raw material-human organs-do not have to be paid for, thanks to federal law. It is one of the great federal subsidies of our time.

How close do you think science and medicine can come to truly defining death — at least as it pertains to the notion of death on the organismal, human level?

Because "irreversible" is locked into our present definition of death, I don't think we can ever truly define it. And what happens to consciousness? I personally believe it disappears with the entropy of the body, but many would say otherwise, and I have no evidence to dispute them.

To what extent is the definition of death a philosophical question? With your answer to that question in mind, do you think doctors are the most well-equipped people to come to decisions involving life and death, and are there any realistic alternatives?

Today, more than ever, it has become a philosophical question. When we witnessed the brain-death exam of a woman in Springfield, Massachusetts, after the perfunctory tests, a nurse proclaimed, "Whatever it was that made her her isn't there anymore." So that is the new standard: "personhood." Doctors say brain death is imperfect as a rule for calling the body biologically dead, but so what? The "person" is gone.

Here's what I'd like to know: during what year of medical school do doctors learn what a "person" is, and when the person is missing?

In your experience, how do non-scientists and non-doctors relate to the concept of death? Is there as much disagreement over (or differences in definition between) what it means for a cell, organ, animal, or human to be dead, or is that kind of thing not even on the average person's radar?

I imagine there are grave disagreements. But I think on a personal level, we are all on the same plane. We are terrified. Death is the unknown and shall always be the unknown. We don't know where we're going, if anywhere. And will we be annihilated? I kind of think so, and that is hard to grasp. We all have to face the fact that at some point, the universe will exist without us. We are not special and are not ultimately needed. Neither science nor medicine is of much help here, and neither is most egocentric religion. I've had to deal with this reality during the ten years I worked on this book. In the end, it is of some comfort to know that whatever inequities exist in the world, we are all equal in this regard, and we all share the same fate.